A threshold valve is a valve having an inlet port and an outlet port and normally blocking flow between these ports. Only when the pressure at the inlet port exceeds a predetermined maximum does such a valve open. Normally the valve is set up so that it closes again when the pressure at its inlet port drops below a predetermined minimum that is normally somewhat lower than the maximum pressure at which it opens originally. Most hydraulic and pneumatic systems are provided with this type of valve to prevent overpressurization of any component.
It is normally desired that such a valve be of a so-called full-flow type. Thus when open such a valve should present very little resistance to flow so that the overpressure condition can be remedied rapidly. Thus the maximum pressure at which the valve opens is normally quite a bit higher than the minimum pressure at which it again closes, and prior to dropping of the inlet pressure to this minimum pressure the valve should remain fully open.